A calcium- and activator-dependent cGMP phosphodiesterase has been purified to apparent homogeneity from bovine brain supernatant. The anomalous kinetics of cGMP hydrolysis as well as the effects of disulfide reductants, "hydrophobic" agents (imidazole, ethanol) and protein "stabilizers" (glycerol, ovalbumin) may suggest that basal and activated activity may reflect states of aggregation/dissociation of the enzyme's subunit structure.